Effects of Spruce Decline on the Blackpoll Warbler

January 1, 2001 by Russ Greenberg

When nest building, many species of forest-dwelling birds show distinct preferences for particular species of trees. The decline and disappearance of those tree species due to disease, pollution, climate change, or other factors, may therefore affect avian nesting ecology and population dynamics.

In montane forests of the northeastern U.S., populations of red spruce (Picea rubens) have been declining by up to 85% since the 1950's as a result of acid precipitation. We examined the potential effects of spruce decline on the nesting ecology of the blackpoll warbler (Dendroica striata), a spruce woods specialist whose populations are reportedly declining.

The blackpoll warbler is a nearctic-neotropical migratory songbird that breeds in boreal spruce and fir forests of Alaska, Canada, and the northeastern United States. Breeding Bird Survey (BBS) data indicate that southeastern populations are declining at rates up to 1.5% per year, while western populations are apparently stable or increasing.

Blackpoll warblers prefer red spruce

In the eastern U.S. and southeastern Canada, the range of Blackpoll Warblers mirrors that of red spruce, one of its preferred nesting trees. In many parts of its range, red spruce populations are also declining as an indirect result of acid deposition.

We hypothesized that southeastern blackpoll warbler population declines are linked to red spruce declines. In 2001, we found 23 blackpoll warbler nests at 4 sites in the Green Mountains of Vermont.

Blackpoll warblers showed a strong preference for spruce among conifers in the habitat. Over one-third of Blackpoll nests were placed in red spruce, whereas ca. two-thirds of nests were placed in balsam fir. That use of red spruce was more than double red spruce availability in the habitat suggests strong preference for spruce by Blackpolls.

Nests in spruce were more successful than nests in balsam fir. They produced more young than did nests in balsam fir.

Average Number of Fledglings

Lower nesting success in red spruce implies that the decline of spruce from eastern montane forests may be related to population declines of blackpoll warblers in the same region. Loss of tree species from otherwise intact forests may be an important factor regulating populations of forest-breeding birds.